Genetic engineering of plants has enabled the production of plants having improved characteristics or traits, such as disease resistance, insect resistance, and herbicide resistance. New genes can be expressed in the plant cell to exhibit the desired phenotype such as a new trait or characteristic.
The proper regulatory signals, such as promoters, must be present and be in the proper location with respect to the coding sequence of a gene in order to obtain expression of a gene product inserted into a plant cell. For endogenous genes, a promoter is a DNA sequence that directs cellular machinery of a plant to produce RNA from the contiguous coding sequence downstream (3′) of the promoter. The promoter region can influence the rate, developmental stage, and cell type in which the RNA transcript of the gene is made.
Expression of heterologous nucleotide sequences in a plant host depends in part upon the presence of an operably linked promoter that is functional within the plant host. The promoter sequence chosen is based on when and where within the organism expression of the heterologous nucleotide sequence is desired. Where expression in specific tissues or organs is desired, tissue-preferred promoters can be used. Where expression in response to a stimulus is desired, inducible promoters can be used. An inducible promoter is a promoter that is capable of directly or indirectly activating transcription of one or more DNA sequences or genes in response to an inducer. In the absence of an inducer, the DNA sequences or genes will not be transcribed, or will be transcribed at a level lower than in an induced state.
In the case of pest resistance, it is desirable to have a promoter that is induced by plant pests, including plant insect pests, nematodes or disease agents such as a bacterium, virus or fungus. For example, contact with a pathogen, or some other signal associated with the pathogen, induces activation of transcription, such that a pathogen-fighting protein or nucleic acid will be produced at a time when it will be effective in defending the plant. A pathogen-induced promoter may also be used to detect contact with a pathogen, for example by expression of a detectable marker, e.g., so that the need for application of pesticides can be assessed.
Soybean [Glycine max (L.) Merr.] is the most widely grown legume in the world, providing an important source of protein and oil. Soybean can be used in many ways such as ingredients in the formulation of a multitude of human foods, animal feed, and industrial products. As the dominant oil-seed in world trade, soybean contributes to greater than about half of global oilseed production. Therefore, soybean is considered as one of the most important economic crops both in the U.S. and abroad. Although global soybean production has increased steadily, future demand for soybean still cannot be satisfied due to a growing world population and limited land resources.
During its entire life cycle, soybean may be attacked by many pathogens, such as fungi, bacteria, viruses, and nematodes, and suffer many diseases in any tissue. The soybean cyst nematode (Heterodera glycines, SCN) is the pest that causes the most economic damage of soybean in the U.S. SCN is a small plant-parasitic roundworm, and most stages of SCN cannot be seen by unaided eyes. SCN feeds on the soybean roots and robs nutrients from the soybean. When soybean plants are severely damaged by nematodes, they become stunted and turn chlorotic. Controlling SCN in commercial soybean productions remains difficult because SCN has a short life cycle and populations can build rapidly. Frequent changes in population virulence of SCN also contribute to the difficulty in the management of this pest. In addition, the cysts of SCN can survive in the soil for up to nine years and then break to release the eggs under proper conditions, increasing the probability of the nematodes' dispersing via infested soil.
The methods used to control and manage SCN in soybean production include crop rotation, the use of SCN-resistant cultivars, and the application of nematicides, which are often used in an integrated manner. However, these approaches often face economic restrictions, are time-consuming, and use of nematicides can result in environmental problems. Additionally, for some areas, economic factors may limit the use of crop rotation. Considering the long-term demands of soybean, it is critical to manage SCN infestation in soybean production. Thus, in implementing a transgenic approach to pest control, one strategy is to increase the expression of desirable nucleic acids and protein in response to pathogens. Consequently, there is a continued need for the controlled expression of nucleic acids and proteins deleterious to pests, for example in response to plant damage.